1. Field of the Invention
The present invention relates to a method of producing a substrate for magnetic recording media in which a NiP plating film has been formed on the surface of an Al alloy substrate.
2. Description of Related Art
In recent years, the improvements in the recording density of magnetic recording media that are used in a hard disk drive has been dramatic. In particular, since the introduction of a magnetoresistive (MR) head or a partial response maximum likelihood (PRML) technique, the increase in surface recording densities has become even more dramatic, and the more recent introduction of a giant magnetoresistive (GMR) head, a tunnel magnetoresistive (TMR) head or the like has meant that recording densities continue to increase at a pace of about 1.5 times a year.
There are still strong demands for even higher recording densities for these magnetic recording media, and in order to satisfy these demands, higher coercive force and higher signal to noise ratio (SNR) of the magnetic recording layer and higher levels of resolution are required.
Further, in recent years, concurrently with the improvements in linear recording density, efforts are also continuing into raising the surface recording density by increasing the track density. For this reason, with regard to the substrates used for magnetic recording media, smoother substrates with fewer scratches have been demanded more than ever before.
Al alloy substrates and glass substrates are mainly used as such substrates for magnetic recording media (namely, disc substrates). Of these, as compared to glass substrates, Al alloy substrates exhibit higher toughness and can be produced more easily, and are thus used for magnetic recording media having a relatively large diameter.
In addition, Al alloy substrates are generally produced through the following steps. First, an Al alloy plate having a thickness of about 2 mm or less is punched out into a doughnut shape to form a substrate with a desired size. Subsequently, the punched substrate is subjected to a chamfering process for the inner and outer diameters and a turning process for the data surface, followed by a grinding process using a grindstone in order to reduce the levels of surface roughness and swelling after the turning process. Thereafter, NiP plating is applied to the substrate surface in order to provide surface hardness as well as to suppress surface defects. Then, a polishing process is conducted on both sides (data surfaces) of the substrate where this NiP plating film has been formed.
Incidentally, in the polishing process for the Al alloy substrates described above, in view of improving both the surface quality (in terms of smoothness and the number of scratches) and the productivity, a multi-stage polishing system involving two or more stages of polishing steps using a plurality of independent grinders has been employed in many cases.
In the polishing step (also referred to as a rough polishing step) at an initial stage in this multi-stage polishing system, in view of productivity, polishing is conducted using abrasive grains such as alumina abrasive grains having a relatively large particle size so as to achieve a high polishing speed. On the other hand, in the final polishing step (also referred to as a finish polishing step) in the multi-stage polishing system, in order to satisfy the requirements to reduce the levels of surface roughness and swelling and the number of scratches, polishing using colloidal silica abrasive grains is generally conducted.
However, when alumina is used as abrasive grains, since alumina abrasive grains exhibit considerably high hardness compared to Al alloy substrates, alumina abrasive grains stick deep into the substrate to cause various problems. For example, these alumina abrasive grains that have been stuck are difficult to remove in the following polishing step, and when they are detached, substrates are damaged by these detached alumina abrasive grains.
As described above, in the multi-stage polishing system, the polishing amount of substrates reduces as the stage progresses and also the abrasive grains included in the abrasives become softer and smaller in terms of particle size. For this reason, the abrasive grains that have been stuck in the former polishing step are difficult to remove in the latter polishing step, and when the abrasive grains that have been stuck are detached to cause damages to the substrates, these damages are difficult to eliminate in the latter polishing step.
For this reason, it has been proposed to use a polishing liquid composition containing both alumina abrasive grains and silica abrasive grains as a polishing liquid composition capable of reducing the sticking of alumina abrasive grains during polishing of Al alloy substrates (refer to Patent Document 1).
In those cases where the polishing liquid composition described in Patent Document 1 is used, since the alumina abrasive grains that have been stuck to the substrates are removed by the silica abrasive grains, it is possible to remove the alumina abrasive grains that have been stuck to the substrates to some extent. However, as long as this polishing liquid composition is used, there is a possibility that the alumina abrasive grains included in the abrasives would stick into the substrates. In addition, since this polishing liquid composition contains both alumina abrasive grains and silica abrasive grains, a high polishing performance exhibited by the alumina abrasive grains cannot be fully utilized, thereby reducing the polishing speed.
Further, in order to reduce the cost for producing substrates, it is required to reduce the number of polishing steps conducted in the multi-stage polishing system. In Patent Document 2, a polishing method employing multiple types of slurries in one grinder has been described.    [Patent Document 1] Japanese Unexamined Patent Application, First Publication No. 2009-176397    [Patent Document 2] Japanese Unexamined Patent Application, First Publication No. 2000-280171